Photographic processes for obtaining a direct positive image without the necessity of a reversal processing step or negative film are well known.
Processes for forming a positive image on a direct positive silver halide photographic material which have been heretofore known can be roughly divided into two types.
In one of the two types of processes, a silver halide emulsion which has previously been fogged is used. Solarization or the Herschel effect is used to destroy the fogged nuclei (latent image) in the exposed portions so that a direct positive image is obtained after development.
In the other type, a internal latent image type silver halide emulsion which has not previously been fogged is used. After being imagewise exposed to light, the emulsion is surface-developed after or while being light-fogged so that a direct positive image is obtained.
The term "internal latent image type silver halide photographic emulsion" as used herein means a silver halide photographic emulsion of the type which contains light-sensitive nuclei mainly in the inside of silver halide particles so that a latent image is formed mainly in the inside of the particles upon exposure to light.
The latter type of process generally provides a higher sensitivity than the former does and is thus suitable for high sensitivity applications. The present invention relates to the latter type of process.
In this art, various methods have heretofore been known. Examples of such methods are described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 2,497,875, 3,761,266, 3,761,276, and 3,796,577, and British Patent Nos. 1,151,363, 1,150,553, and 1,011,062.
With these known methods, a relatively high sensitive photographic light-sensitive of direct positive type can be prepared.
Details of the formation mechanism of a direct positive image are described in, for example, "The Theory of The Photographic Process" (edited by T. H. James, Vol. 4, pp. 183 to 193, Chapter 7), and U.S. Pat. No. 3,761,276.
According to the theory set forth in the above publications, the first imagewise exposure to light produces a so-called internal image in the inside of the silver halide which causes a reduction in the sensitivity of the surface thereof. This in turn causes a selective production of fogged nuclei only on the surface of unexposed silver halide particles. When the light-sensitive material is then subjected to ordinary so-called surface development, a photographic image (direct positive image) is formed on the unexposed portions.
Two fogging processes for selective production of fogged nuclei are known: (1) A process commonly referred to as the "light fogging process" in which the entire surface of a light-sensitive material is subjected to a second exposure to light (as described in British Pat. No. 1,151,363) and (2) a process commonly referred to as the "chemical fogging process" in which a nucleating agent is used. The latter process is described in Research Disclosure No. 15162 (Vol. 151, November, 1976, pp. 72 to 87).
In accordance with the chemical fogging process, the formation of a direct positive color image can be accomplished by a process which comprises subjecting an internal latent image type silver halide photographic material to surface color development after or while subjecting it to fogging, and then subjecting it to bleach and fixing (or blix). The bleach and fixing step is generally followed by rinsing and/or a stabilizing step.
However, the above process has the disadvantage that the light-sensitive material is sensitive to temperature and pH fluctuation of the developing solution during the process or the like in the presence of a nucleating agent. This causes fluctuation in maximum image density. This in turn causes fluctuation in color reproducibility.
On the other hand, the light fogging process does not require as high a pH as the chemical fogging process does. Therefore, the light fogging process has an advantage over the chemical fogging process. However, the light fogging process is as much subject to temperature and pH fluctuation of the developing solution as the chemical fogging process. Accordingly, it is difficult to obtain consistent results with the light fogging process.
U.S. Pat. No. 3,708,298 refers to the incorporation of a fog inhibitor such as triazoline-thione and tetrazoline-thione compounds in a light-sensitive material on which a direct positive image is to be formed in a light fogging process. However, this process is not able to attain high maximum image density, high image speed, and constant image stability.
Thus, no methods have heretofore been known for providing a stable direct positive color image having high maximum color image density and low minimum image density with a low pH (i.e., less than 12) stable developing solution in a short processing time.
Another disadvantage with these methods is that the higher the sensitivity of the direct positive emulsion, the more re-reversal negative images are produced at high intensity exposure.